Method for the production of thin-walled steel components and components produced therefrom

ABSTRACT

The invention relates to a method for the production of thin-walled steel components and similar components, comprising an inner core layer (B) and an external boundary layer (A). Said layers are, at least partly, differently annealed. According to the invention, the disadvantages of conventional roll-cladding and case-hardening processes may be overcome by the following methodology: bonding core and boundary layers made from differently annealed steel alloys, in a casting process to give a combined material with flat alloy gradients on the boundary surfaces, moulding the composite material to the dimensions of the thin-walled components, annealing the components by heat treatment, whereby the layers made from the differently annealed steel alloys obtain different annealing properties.

[0001] The invention in question refers to a procedure for theproduction of thin-walled parts made of steel, which show an inner corelayer and outer boundary layers, in which the layers possess differentlytempered steel alloys and are at least partly differently tempered.Furthermore the invention covers thin-walled parts made of steel with aninner core layer and martensitic hardened outer boundary layers.

[0002] Thin-walled parts made of steel with a wall thickness of lessthan 4 mm, for which a particularly high stress resistance is demanded,for instance in mechanical engineering and vehicle engineering, arefirst of all thermoformed and/or cold-coiled, machined metal-cutting ornon metal-cutting and then tempered by a thermal treatment, namelytempered martensitic or bainitic. Out of hardened steel a part ariseswith continous, uniform, high hardness along the complete cross sectionwhich though has a low toughness. A more favourable combination of wearresistant surfaces with high toughness in the inner zone is achieved bythe use of case hardened steels. By a carbonizing treatment in athermochemical hardening process tempered, hard surface layers areproduced while furthermore the inner core keeps a high toughness. Arelatively effortful production procedure contrasts with theadvantageous use qualities, however. A distortion of hardening is namelyunavoidable through the relatively long case-hardening time of forexample 180 minutes at 850-950 degrees Celsius and the followingquenching in the oil bath or in the gas current. This causes measure andform deviations which require an effortful subsequent treatment whichquite considerably increases the production effort and expense. Inaddition, there is a relatively rough hardness structure which has anaustenitic grain size according to DIN 50601 of for example 5 or 6. Aninclination towards intercrystalline failures arises from it at theintercrystalline grain boundaries.

[0003] As a substitute for the case hardening, furthermore the use isknown of roll-bonded steel in which two or more, different alloyed tapesor panels get together rolled preferably from cold tape. By the pressureand the temperature the core and surface layers of different alloyedsteels are connected intimately with each other at the surfaces in theroll gap. The metallic compound arises from the following anneal bydiffusion events. Such a roll bonding procedure is indicated in the DE41 37 118 A1, for example. An abrupt, volatile changeover arises fromit, however, between the different material layers. The hardnesstransition between layers tempered and not tempered is also thereforeappropriately steep so that due to the load induced tension gradientsrelatively thick surface layers must be produced. By the relativetensions the latent danger insists at the contact surface moreoverunavoidably that the peripheries at use chip off by transgression of theapparent yielding point in the joint area. This disadvantage can, asmentioned above, merely be met by surface layers being dimensioned morethickly, what, however, in turn leads to an unwanted higher wallthickness of the parts and moreover makes the production more difficult.

[0004] The DE 196 31 999 A1 has already been suggested certifiable forthe production of composite sheet metals in a continuous castinginstallation by casting together core and surface layers. Through this asteel layer material shall be produced. The difficulties at theproduction of layers which are differently tempered or hardened aren'ttaken up, however. A similar continuous casting procedure is mentionedin the DE 33 46 391 A1 at which layer sheet metals are also embedded ina melt. The difficulties at the realization of layers which aredifferently tempered or hardened also aren't mentioned, however, intothis. The aforementioned continuous casting procedures and installationsare obviously moreover alone suitable for the production of relativelythick blanks or sheet metals and not for the production of thin-walledparts. It similarly behaves with U.S. Pat. No. 3,457,984 for theresulting level for the technological development. This refers, tojacket the casting rope of a continuous casting installation with sheetmetal to it merely.

[0005] In view of this one the formulation arises for the invention onhand, to indicate an efficient procedure for the production ofthin-walled parts made of steel with different tempered, especiallydifferently hardenable layers. Furthermore a part with tempered, thatmeans hardened layers shall be provided, which has improvedcharacteristics and can be produced particularly more economically thantill now in case of the reduced effort.

[0006] The procedure according to the invention provides the followingprocedure steps:

[0007] connecting of core and surface layers from differently temperablesteel alloys in a casting procedure to a composite material with alloygradient going flatly at the interfacials,

[0008] deforming of the composite material on the size of thethin-walled parts,

[0009] tempering of the parts by heat treatment in which the layers ofthe differently temperable steel alloys show different tempercharacteristics.

[0010] The procedure according to the invention shows the advantage tocombine core and surface layers with each other of steel materials withdifferent temper characteristics, namely particularly differentmartensitic hardenability qualities so that thin-walled parts whichunite the respective advantages of the case hardening and the rollbonding into themselves are made available.

[0011] In detail a strength distribution is caused by the temperingaccording to invention with respect to the strength and/or hardnessqualities of the composite material which is comparable with thecase-hardening course considered special generally advantageously.Unlike case-hardening however practically no delay appears at theprocedure according to invention, so that a precise, measure and formexact part is made available without measure corrections being required.Furthermore accordingly to the invention the predefined, flat alloygradient at the interfacials between the layers is avoiding theformation of inner material notches as they are unavoidable at the rollbonding process, as mentioned at the beginning. Thanks to the hardnessand strength gradient optimized by this no more danger insists that thesurface layers chip off by exceeding the tensile strength in the jointarea, that is at the interfacial, at a high load tension.

[0012] Preferably the individual layers of steel alloys are providedwith different martensitic hardenability qualities, i.e. differentcontents of carbon, chrome and manganese, in which the followingtempering is made by martensitic or bainitic tempering, that means aheat treatment with the steps heating up-quenching-tempering. Inparticular these tempered layers consist of higher alloyed, that meanscarbon richer steel as that of the layers not temperable. In this case acarbon gradient going appropriately flatly is realized in the area ofthe alloy gradient going flatly. This transition zone between carbonricher and carbon poorer layers extends at a wall thickness of the partsof less than 4 mm at less than 20%, preferably less than 15% of the wallthickness. In any case the area of the flat alloy or carbon gradient isbroader than 0.1 mm that is around more than a range broader as at theknown roll bonding procedure.

[0013] Preferably the tempered layers form the surface layers of theparts, which through this get hardened in their surface and get ahardness course which is approximately similar to that by casehardening. The disadvantage of the case hardening, that due to the longresidence time a relatively rough grain structure appears in theperipheries which leads to an increased microcrack sensitivity, isavoided by the layer order according to the invention, however. Throughrelatively short residence times, a wear resistant fine grain structurewith high toughness also results namely in the surface layer in theperiphery which leads to a particularly little microcrack sensitivity.Preferably parts can be produced by a procedure according to inventionwith a wall thickness of less than 4 mm. These tempered layers, thatmeans the martensitic hardened layers, have a crosscut part of about 10%to 50% of the wall thickness. Alternatively the core layer of the partscan be tempered, for example hardened, while the surface layers consistsof not temperable steel alloys or stainless steels.

[0014] The tempered layers made of materials as for example C 55, C 67or other steels of the EN, 100 Cr 6 or X 20 Cr13, X 35 CrMo 17 formpreferably the boundary layers, while the core layers are made ofmaterials not temperable as for example DC 01 or C 10. For certainapplications these temperable layers can also form the central layers,however, for example a spring steel core made of C 60, C 67 or C 75,while the surface layers consist of well deformable steels such as C 10or DC 01 or also of stainless steels like X 5 CrNi 1810.

[0015] The alloy gradient according to the invention between the surfaceand the core layers can be provided in such a manner, that for theproduction of the composite material for the surface layers blanks areordered parallel by far to each other made of steel hardenablemartensitic and the core layer situated in between is spilled withfused, carbon poorer steel. For providing of the surface layers, forexample cold or surface treated warm tape is used with predefinedchemical analysis, particularly large carbon amount. By the corematerial spilled fusably in between this which has a lower carboncontent there happens a local on-glaze of the blanks at the materialinterfaces, through what due to diffusion processes a flat alloy orcarbon gradient with a depth of about 0.1-0.3 mm is formed. Thesequalities are made possible by the connection according to invention bymeans of a final dimension near casting procedure.

[0016] The blanks preferably are cooled from outside by the castingwheels or the chill form when casting of the fused core material. Bythis fact even at thin blanks the breadth of the alloy gradient can besteered so that it is in the area of 0.1 mm and is up to 10% of thecomplete crosscut.

[0017] It is particularly of great advantage if the blanks are broughtas steel hoops to the edge of the casting gap of a casting plant workingcontinuously. The casting plant can alternatively be a rope castingplant with a firm open-ended mould or can be equipped for the executionof a continuous process of casting and rolling with rotating rolls(casting wheels) limiting a casting gap. According to the invention thetape which forms the surface layers becomes introduced to the rolls orcopper jaws on the edge of the glaze marsh into the casting gaplengthways on both sides. The tapes must be bright, free of cinder andoxide as well as if necessary roughened by a corresponding surfacetreatment at least on her insides where the liquid core material iscasted.

[0018] To stop an unwanted oxidation of the wall surface by the warmingat the supplying into the casting gap, it is advisable to supply thesteel hoops coming in or the blanks under an oxidation protecting cover.Preferred this can be a protective gas atmosphere. Such a protective gasbell is produced by supplying of inert gas respective mixtures of inertgases.

[0019] As soon as the melting of the core material comes in contact withthe surface of the tape, this is heated on about 950 degrees Celsius, sothat a metallic joint arises by the diffusion bonding of the meltingwith the surface of the tape with the flat alloy gradient according toinvention.

[0020] By the tape (warm tape) forming the surface layers the warmth isfurther given to the copper rolls or to the wall of the chill form sothat the tapes do not melt on completely what would not be desired.Result of this casting combination in the final dimension near range ofthe wall thickness is an increase of the casting performance since thewarmth removal is made by the on-heating of the supplied surface layers,this means that the casting gap is cooled by the supplied, coldmaterial.

[0021] A hot-rolling process preferably follows the aforementionedcasting. In case of the temperatures above 950 degrees Celsius it isguaranteed due to the high surface pressure and deformation that acomplete binding of the layers is certainly achieved in the wayaccording to the invention, and himself then to be more precise if themetallic joint shouldn't have been sufficient at the contact of themelting with the tape surface. At the latest, it then develops a flatmaterial transition gradient between the layers, which amounts in aregion of about 0.1 mm. The surface of the rolling stock gets a statepoor of roll marks and tinder without flame chipping or blackoperations.

[0022] The composite material is then rolled out by warm and/or coldrollers with an rolling ration of regularly more than 30% to a thicknessof 1 to 5 mm. Preferably by following cold rolling the least formingcoming up to requested dimensions of the wall thickness of the parts,which amounts in a region of about 4.0 mm, in which the surface showslowest fault depths and high pore liberty, which is the prerequisite forthe later use for highly stressed components, for example enginecomponents. If necessary multiple cold rolling and process-anneals canbe required for the definite contouring.

[0023] Before the further processing by bending, pressing or somethingelse the composite material rolled on measure is subjected preferably toa recrystallization annealing or soft anneal on about 730 degreesCelsius. In this soft annealed state the composite material is suitedwell for the cold forming, for example of engine components.

[0024] Finally the composite material formed on measure is subjected forthe tempering to a heat treatment in which is carried out a martensitichardening of the temperable layers. The differently hardening layers,for example the surface layers, are martensitically hardened by thesequence of the procedure steps heating up-quenching-tempering known tohimself, while the areas less alloyed show lower hardness andfurthermore keep their toughness.

[0025] By means of a partial heat treatment, for example by means oflaser or electron ray treatment a locally restricted tempering, thismeans hardening, can take be achieved. A tempering can alternatively becarried out in the short time run procedure, prefers in a protectiveatmosphere furnace. This makes possible a particularly efficientproduction of function optimized strip stock and components.

[0026] A part which is produced according to the aforementionedprocedures and thin-walled with a soft core layer and martensitic,hardened surface layer which consists of a cold formed, hardenedmultilayer composite material, which has carbon enriched, martensiticlyhardened surface layer and relatively to this a carbon poorer corelayer, in which the carbon gradient goes flatly between the layers, hasparticularly advantageous application possibilities. This part accordingto the invention stands out by the fact that it gets close to acase-hardened steel part with regard to hardness course and strengthdistribution. Material qualities which are not attainable with otherhardness procedures can, however, be provided by the use of a multilayercomposite material from different hardening destitute of martensiticlayers. Thanks to the flat transition zone an adjustment of thecomparison tension conditions is given to the load tension curve in thecrosscut. A more efficient production correspondingly arises in thecontext of optimized function qualities, such as a surface free of poresand decarburization without edge oxidation of the grain boundaries withan Austenit-grain size finer than 8 according to DIN 50601.Alternatively the part can own surface layers not temperable for examplefrom stainless steel alloys, and a tempered core layer either, forexample made of spring steel.

[0027] The of the part according to the invention is preferably up to4.0 mm. The carbon gradient in the transition zone extends in the regionof about 10 to 30% of the wall thickness, therefore in any case aboutmore than 0.1 mm.

[0028] The materials for the surface and core layers are coordinatedwith each other preferably so that the hardness of the core layercorresponds to at least 30% to 50% of the hardness of the surfacelayers.

[0029] The part can consist of two different materials, for example alowly alloyed core layer and highly alloyed surface layers. The chemicalcomposition of the surface layers also can, however, be different whenrequired so that at least three layers are existing altogether withdifferent material qualities. Through this can be reached a furtherimproved function optimization of the parts like anti-corrosionprotection or the possibility of fusion welding.

[0030] Furthermore with parts being produced according to the inventioncan be realized asymmetrical spring ways or self adjusting spring waysor spring strengths.

[0031] Broader features and advantages of the invention on hand getclear with the following description of preferential execution examplesunder reference to the enclosed illustrations. Pointing into this

[0032]FIG. 1—shows a crosscut through a Part according to the invention;

[0033]FIG. 2—shows a schematic representation of a casting plant for theproduction of strip stock according to the invention.

[0034]FIG. 1 shows a cut through a cold formed part 1 with a martensitichardened surface layer. This is preferably formed of strip stock with acomplete thickness 5 which lies in the area of 0.3 to 4.0 mm.

[0035] The represented part consists of steel layer material withseveral layers. These cover in particular a core area B from a carbonpoor alloy and surface layers A of a carbon rich, martensitic hardenedsteel. The core layer B consists for example of Ck10, DC01, C 10, C 35or C 53. The outer surface layers consist for example of Ck67, C 55, C67 or also 102 Cr6, x5 Cr Ni 1810 or something like that. The surfacelayers A also can for their part consist of steel alloys withrespectively different analyses.

[0036] The unusual feature of the represented part 1 lies into this thatthe layers A, B, A have already been connected to each other before thecold massive forming on the final measure 5 in accordance with theprocedure according to the invention so that at the layer borders broadtransition zones G which are indicated hatchedly and in which a flatcarbon gradient has developed by carbon diffusion between the shiftmaterials which lies in the area of several {fraction (1/10)} mm.

[0037] The complete part 1 (FIG. 1) after for example being cold formedto an engine component has been subjected to a martensitic hardeningprocess. The surface layers A have hardened through this while the coreB keeps a relatively big toughness. Thanks to the flat carbon gradient Gaccording to the invention a flat tension curve exists at the layerborders so that no danger of chipping off of the surface layers from thecore layer B exists as this exists for example in the case at theroll-plated tape in accordance with the level of technology.

[0038] Practically no hardness delay occurs at martensitic hardening,this means no unwanted form and measure change so that the part 1 can betaken to the final measure 5 already before the hardening process and noafter-work is required as this has to be done by the way ofcase-hardening. By the choice of the layer materials an advantageousstrength and hardness course is however reached which is comparable orbetter as with the case hardening. The through-hardening of the surfacelayers A at the layer material according to the invention can be carriedout namely with a short time heat treatment, that is with a considerablyshorter time for Austenitising as by the way of case-hardening. By thisthe surface layers A get a more fine-grained hardness structure than itwould be attainable by case hardening. A possible crack growth is notstamped intercrystallinely but transkristallin and corresponding a clearimprovement on the toughness and according an increase of the life timearises.

[0039] Alternatively the component 1 according to the invention can alsoown a tempered core layer B in accordance with FIG. 1, that has beenhardened particularly martensitic or bainitic, and surface layersrelatively tempered not or less to this, in which it is formed of a coldformed tempered multilayer composite material, which owns a carbon richcore layer B, which is tempered, and relative to this carbon poorersurface layers A, in which the zone of the carbon gradient's G, asexplained previously, is positioned between the layers A, B.

[0040] Particularly interesting material matings for the production ofspring elements are conceivable tempered spring steel in the core andcorrosion poor, for example stainless alloys in the surface layers A.Through this an asymmetrical spring way or a self adjusting spring forcecan be provided, for example.

[0041]FIG. 2 shows schematically a continuously working casting androlling plant with two rolls. This shows two rotating, water-cooledcopper rolls 2 which limit a casting gap with 1-5 mm of breadth. Fromabove the glaze marsh 3 is pressurized with glaze liquid material B overa diving tube 4. Along the edges of the casting gap A strip stock isbrought to by stock coils. With the core material B spilled in thecasting gap the connection takes place there between the material Asupplied as a steel hot strip and the material B supplied glazeliquidly. An optimal metallic joint is made in any case by hot rollingby the high surface pressure at temperatures of above 950° C.

[0042] In the represented plant the warmth removal along the copperrolls 2 takes care by the steel hot strip A that the carbon gradient Gof the steel hot strip A does not succeed too far. In any case anadequately thick surface layer of the carbon rich, hardenable ofmartensitic edge material A remains available with which parts can berecieved in the following thermal tempering and hardness procedures withthe represented hardness course or the strength distribution.

[0043] With the represented plant according to the invention steel layermaterials can be produced with extremely different qualities regardingthe tempering of the individual layers. The cold deformable compositematerial can already well and efficiently be processed particularly tofinal measure. Unlike the known procedures neither it comes at hardeningfollowing this to an adverse hardness delay nor there is the danger ofthe chipping off of surface layers. These show namely a fine, toughhardness structure which does not lead to the rupture of the part evenat severe use or short time overload.

1. Procedure for the production of thin-walled parts made of steel,which show an inner core layer and outer surface layers, in which thelayers are at least partly differently tempered, characterized by theprocedure steps: connecting of core and surface layers from differentlytemperable steel alloys in a casting procedure to a composite materialwith alloy gradient going flatly at the interfacials, deforming of thecomposite material on the size of the thin-walled parts, tempering ofthe parts by heat treatment in which the layers of the differentlytemperable steel alloys show different tempering characteristics. 2.Procedure according to claim 1, characterized in that the layers havedifferent martensitic hardenability qualities and the tempering iscarried out by martensitic hardening.
 3. Procedure according to claim 1or 2, characterized in that the tempered layers consist of higheralloyed steel than the not tempered layers.
 4. Procedure according toone of the claims 1 to 3, characterized in that the core layers and thesurface layers include tempered layers and stainless layers. 5.Procedure according to one of the claims 1 to 4, characterized in thatthe tempered layers form the surface layers.
 6. Procedure according toone of the claims 1 to 5, characterized in that the tempered layers owna wear resistant fine grain structure with high toughness and littlemicrocrack sensitivity.
 7. Procedure according to one of the claims 1 to6, characterized in that the tempered layers form the core layers. 8.Procedure according to one of the claims 1 to 7, characterized in thatthe parts show a wall thickness of less than 4 mm.
 9. Procedureaccording to one of the claims 1 to 8, characterized in that thetempered layers show a quota of the cross section of 10 to 50% of thewall thickness.
 10. Procedure according to one of the claims 1 to 9,characterized in that the area of the alloy gradient is broader than 0.1mm.
 11. Procedure according to one of the claims 1 to 10, characterizedin that the alloy gradient extends on about 10-25% of the wallthickness.
 12. Procedure according to one of the claims 1 to 11,characterized in that for the production of the composite material forthe surface layers blanks made of martensitic hardenable steel arearranged parallel by far to each other and the core layer situated inbetween these is spilled with glaze liquid, carbon poorer steel. 13.Procedure according to claim 12, characterized in that the blanks arecooled from outside.
 14. Procedure according to claim 12 or 13,characterized in that the blanks are brought as steel hoops to the edgeof the casting gap of a casting plant working continuously. 15.Procedure according to claim 14, characterized in that the rope castingplant has a firm open-ended mould.
 16. Procedure according to claim 14,characterized in that the casting plant has cooled rotating rollslimiting the casting gap.
 17. Procedure according to one of the claims 1to 16, characterized in that the deformation of the composite materialis carried out by a hot-rolling process.
 18. Procedure according to oneof the claims 1 to 16, characterized in that the deformation of thecomposite material is carried out by a cold-rolling process. 19.Procedure according to one of the claims 1 to 18, characterized in thatthe composite material, which is deformed to the measure of the parts issoft-anneal and afterwards deformed to parts.
 20. Procedure according toone of the claims 1 to 19, characterized in that the tempereing iscarried out by a short-time heat treatment.
 21. Procedure according toone of the claims 1 to 20, characterized in that the composite material,which is formed to measure, is treated by a tempering for themartensitic hardening of the temperable layers.
 22. Procedure accordingto one of the claims 1 to 21, characterized in that a locally determinedtempering is carried out.
 23. Procedure according to one of the claims 1to 21, characterized in that the martensitic tempering is carried out ina continuous operation.
 24. Thin-walled part made of steel, inparticular produced according to a procedure according to one or more ofthe claims 1 to 23, with a core layer and surface layers, characterizedin that it consists of a cold-rolled tempered multilayer compositematerial, which owns tempered surface layers and a not tempered corelayer.
 25. Thin-walled part made of steel, in particular producedaccording to a procedure according to one or more of the claims 1 to 22,with a core layer and martensitic hardened surface layers, characterizedin that it consists of a cold-rolled hardened multilayer compositematerial, which owns carbon rich, martensitic hardened surface layersand a relatively carbon poor core layer, in which the carbon gradientgoes flatly between the layers.
 26. Thin-walled part made of steel, inparticular produced according to a procedure according to one or more ofthe claims 1 to 22, with a core layer and surface layers, characterizedin that it consists of a cold-rolled hardened multilayer compositematerial, which owns not tempered surface layers and a tempered corelayer.
 27. Part according to one of the claims 24 to 26, characterizedin that the wall thickness of the part is less than 4 mm.
 28. Partaccording to one of the claims 24 to 26, characterized in that thecarbon gradient extends in the region of about 10 to 30% of the wallthickness of the part.
 29. Part according to one of the claims 24 to 27,characterized in that the carbon gradient extends about more than 0.1mm.
 30. Part according to one of the claims 24 to 29, characterized inthat it owns in the surface zone a wear resistant fine grain structurewith high toughness and little microcrack sensitivity.